Cleanup AudioDeviceThread to reduce locking and unused features.

media/audio/audio_input_device.h

Index: media/audio/audio_input_device.h
diff --git a/media/audio/audio_input_device.h b/media/audio/audio_input_device.h
index 6f4e371a4d88ee00b02337f8cd81716ed57c45fd..dc6c69190a7c0a83214f1825c155f7268e10ab8f 100644
--- a/media/audio/audio_input_device.h
+++ b/media/audio/audio_input_device.h
@@ -156,8 +156,8 @@ class MEDIA_EXPORT AudioInputDevice
   // In order to avoid a race between OnStreamCreated and Stop(), we use this
   // guard to control stopping and starting the audio thread.
   base::Lock audio_thread_lock_;
-  AudioDeviceThread audio_thread_;
   std::unique_ptr<AudioInputDevice::AudioThreadCallback> audio_callback_;
+  std::unique_ptr<AudioDeviceThread> audio_thread_;
 
   // Temporary hack to ignore OnStreamCreated() due to the user calling Stop()
   // so we don't start the audio thread pointing to a potentially freed